Internal circuits in many devices such as computers, printers, liquid crystal displays, etc., use multiple DC voltage levels that must be powered on, and/or off, in a specific sequence in order to ensure proper functionality and to avoid damage to the circuitry. For example, LCD (liquid crystal display) drive circuits often have one or more negative bias voltages, a digital logic voltage and yet another analog power supply. Bringing voltage levels up in an improper sequence in such circuits can result in unintended paths for current flow when parasitic diodes within IC's are biased into conduction by the unbalanced voltage levels. This can cause a latch-up condition, possibly resulting in overheating and eventual component meltdown. The manufacturers of such circuits specify the order in which the various voltage supplies should be energized, and it is up to the designer of the power circuit to find a way to meet that specification.
Various methods of providing power sequencing have been used previously. For example, when the required sequencing of power runs from highest level first to lowest level last, a simple cascading of voltage regulators can be used whereby the output of the regulator supplying the highest voltage rail is used as the input to the regulator supplying the next lowest voltage rail, and so on, down to the lowest voltage level needed. The problem with this solution is that the first regulator in the chain must not only supply the current for its primary voltage rail, but it must also supply the current needed for every lower voltage rail as well. Even with the reduced voltage drop at each successive stage, the additional current draw can quickly require larger, more expensive parts and/or added heat sinking devices. In addition, the reduced voltage drop between stages will likely require the use of more expensive LDO (low dropout) regulators.
Another alternative for sequencing multiple voltage levels is to add an RC filter on the output of a second voltage rail, for example, to slow its rise-time with respect to a first voltage rail. Although quite simple to implement, this solution has a number of drawbacks, including its dependence on the original rise-times of each of the voltage rails, the power and voltage loss in the RC filter resistor, the inability to clamp the delayed second rail voltage if the first voltage rail does not come up, and the second filtered voltage rail being held up longer than the first voltage rail upon powerdown.
Other solutions for sequencing multiple voltage levels include the use of voltage monitor IC's, dedicated voltage sequencing IC's and suitably programmed microcontroller I/O ports. However, these solutions have drawbacks including higher costs, additional space required for active components and increased complexity.